In a fuel injection engine mounted in a motorcycle, an air-intake passage extends from an air cleaner box to a combustion chamber formed in the interior of a cylinder. The air-intake passage typically includes an intake port that is formed in the interior of a cylinder head and allows the interior of the combustion chamber and the outside of the cylinder head to communicate with each other, and an air-intake pipe connected to the intake port and extending outside the cylinder head. A butterfly valve is disposed in the air-intake pipe or at a location between the air-intake pipe and the air cleaner box. A fuel injector is disposed downstream of the butterfly valve in an air flow direction (see Japanese Laid-Open Patent Application Publication No. 2000-320434).
In the above described fuel injection engine, air is supplied from the air cleaner box to the combustion chamber through the air-intake passage with an amount according to an opening degree of the butterfly valve which is moved in response to a rider's operation to rotate a throttle grip of the motorcycle. The fuel injector injects a fuel into the air-intake passage in the form of a mist flow at a time in accordance with an electric signal sent from a controller independently equipped. A large part of the injected fuel is delivered together with the air flowing in the air-intake passage and is supplied to the interior of the combustion chamber from the intake port of the cylinder head.
Some of the fuel injected from the fuel injector is not delivered together with the air but collides against and adheres to an inner wall of the air-intake passage. In many cases, the fuel adheres to an inner wall of the intake port of the cylinder head, though the location varies depending on the placement or orientation of the fuel injector. The fuel adhering to the inner wall is later vaporized into a gas, which is delivered into the combustion chamber together with the air flowing from the air cleaner box through the air-intake passage.
A water jacket is formed in the cylinder head to cool portions of the cylinder head by cooling water flowing therein, during running of the engine. For example, the cooling water flowing in the water jacket formed around the intake port cools a wall portion forming the intake port, thereby cooling the air supplied to the combustion chamber. This increases filling efficiency of the air.
However, if the entire wall portion around the air-intake port is cooled, then vaporization of the fuel adhering to the inner wall of the intake port is impeded. As a result, fuel efficiency of the fuel injection engine is not improved. Such a problem arises in general fuel injection engines as well as in a fuel injection engine mounted in a motorcycle.